The present invention generally relates to an apparatus for the monitoring and control of a wafer temperature in a semiconductor process chamber and more particularly, relates to an apparatus for the real-time monitoring and control of a wafer temperature positioned on an electrostatic chuck by using a plurality of thermoelectric cooling module and/or an optical sensor for sensing a temperature of the wafer.
In the fabrication of modern integrated circuit devices, one of the key requirements is the ability to construct plugs or interconnects in reduced dimensions such that they may be used in a multi-level metalization structure. The numerous processing steps involved require the formation of via holes for the plug or interconnect in a dimension of 0.5 xcexcm or less for use in high-density logic devices. For instance, in forming tungsten plugs by a chemical vapor deposition method, via holes in such small dimensions must be formed by etching through layers of oxide and spin-on-glass materials at a high etch rate. A high-density plasma etching process utilizing a fluorine chemistry is frequently used in the via formation process.
In a modern etch chamber, an electrostatic wafer holding device, i.e., an electrostatic chuck or commonly known as an E-chuck, is frequently used where the chuck electrostatically attracts and holds a wafer that is positioned on top. The E-chuck holding method is highly desirable in the vacuum handling and processing of wafers. In contrast to a conventional method of holding wafers by mechanical clamping means where only slow movement is allowed during wafer handling, an E-chuck device can hold and move wafers with a force equivalent to several tens of torr pressure.
Electrostatic chucking is a technique used to secure a wafer onto a susceptor in a wafer processing chamber. In more recently developed wafer processing technology, the electrostatic wafer holding technique is frequently employed in which a chuck electrostatically attracts and holds the wafer. It is a highly desirable technique used in the vacuum handling and processing of silicon wafers. In contrast to a conventional method of holding wafers by either gravity or mechanical clamping means where only slow motion of the susceptor is allowed during wafer handling, an electrostatic wafer holding device can hold wafers with a force that is significantly higher. Since there are no moving parts acting on the wafer, there are no particle generation or contamination problems in the processing chamber.
Electrostatic chucks have been used to overcome the nonuniform clamping associated with mechanical clamping devices. The electrostatic chuck utilizes the attractive coulomb force between oppositely charged surfaces to clamp together an article and a chuck. It is generally recognized that in an electrostatic chuck, the force between the wafer and the chuck is uniform for a flat wafer and a flat chuck. This is in contrast to a mechanical clamping system where the clamping is effected around the peripheral of a wafer. Special provisions must be made to compensate for the bowing at the center of the wafer caused by the pressure of cooling gas which is pumped in between the wafer and the pedestal that is supporting and cooling the wafer. For instance, in order to compensate for the bowing of the wafer, one solution is to make the pedestal in a domed or bowed shape. This is eliminated in an electrostatic chuck where the wafer is held on a substantially planar chuck surface with an even electrostatic force distributed according to the electrode layout. The electrostatic force is generally sufficient to prevent bowing of the wafer and to promote uniform heat transfer over the entire wafer surface.
In the normal operation of an electrostatic chuck, one or more electrodes formed in the chuck body induce an electrostatic charge on the surface of a dielectric material that is coated over the chuck surface facing the wafer, i.e., between the bottom surface of the wafer and the top surface of the chuck. A typical dielectric material that can be used for such purpose is, for instance, a polyimide material. The electrostatic force between the wafer and the chuck is proportional to the square of the voltage between them and to the dielectric constant of the dielectric layer, and inversely proportional to the square of the distance between the wafer and the chuck, i.e.,
Electrostatic Chucking Force=k(V/d)2
wherein k is the dielectric constant of the dielectric layer. V is the voltage drop across the dielectric film, and d is the thickness of the dielectric layer. The charging/discharging time constant is RC. When R is very large for a thick oxide backing layer (i.e., d is very large), the electrostatic chucking force can be greatly reduced causing the electrostatic chucking of the wafer to fail.
Since the principal of electrostatic chucking is that there must exist an attractive force between two parallel plates, i.e., between the silicon wafer and the susceptor that have opposite electrical charges, the chucking efficiency is not only determined by the bias voltage, the electric constant of the system, the effective distance between the two parallel plates, but also determined by the wafer grounding efficiency. To utilize electrostatic chucking efficiently in a wafer processing chamber, the surface of the wafer should be electrically conductive so that it can be properly grounded.
A typical inductively coupled plasma etch chamber 10 is shown in FIG. 1. In the etch chamber 10, which is similar to a Lam TCP(copyright) etcher made by the Lam Research Corp., the plasma source is a transformer-coupled plasma source which generates high-density, low-pressure plasma 12 decoupled from the wafer 14. The plasma source allows independent control of ion flux and ion energy. Plasma 12 is generated by a flat spiral coil 16, i.e., an inductive coil, which is separated from the plasma by a dielectric plate 18, or a dielectric window on top of the reactor chamber 20. The wafer 14 is positioned away from the coil 16 so that it is not affected by the electromagnetic field generated by the coil 16. There is very little plasma density loss because plasma 12 is generated only a few mean free paths away from the wafer surface. The Lam TCP(copyright) plasma etcher therefore enables a high-density plasma and high-etch rates to be achieved. In the plasma etcher 10, an inductive supply 22 and a bias supply 24 are used to generate the necessary plasma field. Multi-pole magnets 26 are used surrounding the plasma 12 generated. A wafer chuck 28 is used to hold the wafer 14 during the etching process. A ground 30 is provided to one end of the inductive coil 16.
In a typical inductively coupled RF plasma etcher 10 shown in FIG. 1, a source frequency of 13.56 MHZ and a substrate bias frequency of 13.56 MHZ are utilized. An ion density of approximately 0.5xcx9c2xc3x971012 cm3 at wafer, an electron temperature of 3.5xcx9c6 eV and a chamber pressure of 1xcx9c25 m Torr are achieved or used.
In the typical plasma etch chamber 10, a cooling means for the wafer backside is provided in an E-chuck for controlling the wafer temperature during the plasma processing. This is shown in FIG. 2 for the plasma etcher 40. In the conventional plasma etcher 40, E-chuck 42 is provided for supporting a wafer 44 thereon. E-chuck 42 can be constructed of either a metallic material or of a polymeric material. A plurality of ventilation apertures (not shown) are provided in the E-chuck surface such that a cooling gas can be supplied to the backside 46 of the wafer 44 during plasma processing. The plurality of ventilation apertures in the E-chuck 42 is connected in fluid communication with a cooling gas inlet conduit 38 for feeding a cooling gas into the apertures. The cooling gas inlet conduit 38 is in turn connected to a gas supply line 36, a flow control valve 34 and a cooling gas supply 32. The pressure in the cooling gas supply line 36 is monitored by a pressure sensing device 48 which in turn sends a signal 50 to a controller 52. The controller 52, after receiving signal 50 and comparing to a pre-stored value, sends signal 54 to the flow control valve 34 for opening or closing the valve and thus increasing or decreasing the cooling gas supply through the supply line 36, 38 into the E-chuck 42. The amount of the cooling gas that is supplied to the E-chuck 42 is further adjusted by a needle valve 56 and pumped away by a pump 58.
As shown in FIG. 2, the conventional method for controlling the E-chuck temperature and the wafer temperature is ineffective since there is no feedback control loop for achieving an efficient control of the cooling gas pressure that flows through the E-chuck 42. The temperature of the wafer 44 during plasma processing can not be detected and thus, the temperature can exceed a critical limit to cause a detrimental effect on the coating layers on the wafer. For instance, during a plasma etching process conducted on a dielectric layer, the wafer temperature can increase to such an extent that a photoresist layer coated on the wafer starts to flow during the plasma-on period. The lack of precise control on the wafer temperature in a plasma etcher therefore leads to severe processing difficulties and produces low yield of the wafer.
A test conducted and data obtained on an E-chuck equipped with conventional cooling apparatus is shown in Table 1.
As shown in Table 1, the wafer positioned on the conventional E-chuck rises to a significantly higher temperature during plasma-on 50 that it is impossible to control the wafer temperature only by using the backside cooling gas. The backside cooling gas used can be any inert gas. This is the case when the heat-transfer medium flown between the wafer and the E-chuck is a helium gas, with the chamber pressure in the mini-Torr range and the backside helium pressure in the Torr range during the plasma-on state. When the backside helium cooling gas fails to effectively cool the wafer, a photoresist layer coated on the wafer may flow due to the excessive temperature reached during the plasma-on state.
Table 1 shows data obtained in two separate tests of poly and metal etch. Thermal dots are placed on the wafer to measure the maximum wafer temperature at specific locations during the plasma-on state. The results indicate that, for metal etching, a large temperature differential between the E-chuck and the wafer exists, i.e., as high as 33xc2x0 C. During the poly etch process, a smaller temperature difference of 10xc2x0 C. is observed. These data indicates that during the metal etching process, a temperature differential of 33xc2x0 C. must be accounted for, i.e., or must be controlled. When not controlled, the excessively high temperature affects the etch rate, the uniformity of etching, and the etch profile obtained.
It is therefore an object of the present invention to provide an apparatus for controlling a wafer temperature that does not have the drawbacks or shortcomings of the conventional apparatus.
It is another object of the present invention to provide an apparatus for the real-time monitoring and control of a wafer temperature positioned in a semiconductor process machine.
It is a further object of the present invention to provide an apparatus for the real-time monitoring and control of a wafer temperature by utilizing an optical sensor.
It is another further object of the present invention to provide an apparatus for the real-time monitoring and control of a wafer temperature by utilizing an infrared sensing camera for measuring the temperature of the wafer.
It is still another object of the present invention to provide an apparatus for the real-time monitoring and control of a wafer temperature by utilizing a plurality of thermoelectric cooling modules for cooling a wafer platform.
It is yet another object of the present invention to provide an apparatus for the real-time monitoring and control of a wafer temperature by using a heat exchanger, an optical sensor and a controller.
It is still another further object of the present invention to provide an apparatus for the real-time monitoring and control of a wafer temperature by utilizing at least one thermoelectric cooling module, an optical sensor and a controller.
It is yet another further object of the present invention to provide an apparatus for controlling the temperature of a wafer by using at least one thermoelectric cooling module, at least one thermal couple probe and a controller.
In accordance with the present invention, an apparatus for the real-time monitoring and control of a wafer temperature is provided.
In a preferred embodiment, an apparatus for the real-time monitoring and control of a wafer temperature can be provided which includes a wafer platform for holding a wafer thereon; a heat exchanger for flowing a heat exchanging medium into the wafer platform; an optical sensor for sensing a temperature of a wafer positioned on the wafer platform; and a controller for receiving a signal from the optical sensor, comparing to a pre-stored value and sending a signal to the heat exchanger to increase or decrease a flow of the heat exchanging medium.
In the apparatus for the real-time monitoring and control of a wafer temperature, the wafer platform may be an electrostatic chuck (E-chuck). The heat exchanging medium may be a heated or a cooled heat exchanging fluid. The optical sensor may be an infrared sensing camera, and may be mounted directly over the wafer platform.
The present invention is further directed to an apparatus for the real-time monitoring and control of a wafer temperature which includes a wafer platform for holding a wafer thereon; at least one thermoelectric cooling module (TEC module) embedded in the wafer platform; an optical sensor for sensing a temperature of a wafer positioned on the wafer platform; and a controller for receiving a signal from the optical sensor, comparing to a pre-stored value and sending a signal to the at least one thermoelectric cooling module to increase or decrease a cooling effect.
In the apparatus for the real-time monitoring and control of a wafer temperature, the at least one thermoelectric cooling module may be at least six thermoelectric cooling modules arranged in an array and embedded in the wafer platform. The wafer platform may be an electrostatic chuck. The optical sensor may be an infrared sensing camera mounted directly over the wafer platform. The apparatus may further include at least one thermocouple probe mounted in the wafer platform.
The present invention is still further directed to an apparatus for controlling the temperature of a wafer situated on a wafer platform in a semiconductor process machine which includes a wafer platform for holding a wafer thereon; at least one thermoelectric cooling module embedded in the wafer platform; at least one thermocouple probe mounted in the wafer platform; and a controller for receiving a signal from the at least one thermocouple probe, comparing to a pre-stored value and sending a signal to the at least one thermoelectric cooling module to increase or decrease a cooling effect.
In the apparatus for controlling the temperature of a wafer situated on a wafer platform in a semiconductor process machine, the wafer platform may be an electrostatic chuck. The at least one thermoelectric cooling module may be at least eight modules arranged in an array. The apparatus may further include one thermocouple probe mounted juxtaposed to a corresponding thermoelectric cooling module. The process machine may be a plasma assisted process machine, such as a plasma enhanced deposition chamber or a plasma etcher.